The present disclosure relates generally to actuators, and more particularly to spring-actuated deployment actuators suitable for use with ram air turbines (RATs) in aerospace applications.
Modern aircraft often include a secondary or emergency power system that can provide power in the event that power is unavailable from a primary power system. RATs are commonly used for secondary or emergency power systems to provide electrical and/or hydraulic power. A typical RAT is deployable in flight by opening suitable doors or hatches in the aircraft's fuselage. The RAT presents a rotatable turbine to oncoming airflow, which rotates the turbine. Rotational energy (torque) from the turbine is then transmitted to a suitable power conversion device (e.g., generator, pump, etc.) that converts that rotational energy to a desired form for use by the aircraft.
RATs commonly include an actuator assembly with a spring bias mechanism and a hydraulic cylinder. The spring bias member can provide force to move the RAT from a stowed position to a deployed position, when a stow-lock mechanism is released. Larger RATs often utilize an actuator having an external spring, while smaller RATs often utilize an actuator with an internal spring assembly. In addition to moving the RAT itself, the actuator typically opens one or more associated doors or hatches along the aircraft fuselage. The hydraulic cylinder can be used to provide a snubbing function during an end-of-travel portion of the deployment stroke, and/or to retract the RAT from the deployed position to the stowed position.
During deployment, any air loads and/or gravitational (G) loads acting on the system, including during unusual flight conditions, must be overcome to allow the RAT to fully deploy. Some installations, such as those in or near the nose of the aircraft, have curved RAT deployment doors that may create significant opposing air loads roughly mid-way through the deployment process, as the curved doors interact with oncoming airflows.
Current RAT actuators commonly use a combination of disk springs (i.e., Belleville washers) and a soft spring rate helical spring located inside a piston cylinder to deploy the actuator. In these prior art actuators, typically used with smaller RATs, the disk spring stack provides a high force early in the RAT deployment process, and the helical spring provides a much lower force to finish the deployment (see FIG. 3). These springs and an associated actuator piston fill most of the available volume inside the actuator cylinder. But when actuator loading (e.g., due to airflow loading on an associated door) is relatively high approximately mid-way through the deployment stroke, the helical spring may not be able to overcome the loading on the RAT and door assembly to fully deploy the RAT assembly because the disk spring load falls off to zero or near zero before the mid-way point in the deployment stroke.
Thus, it is desired to provide a relatively highly energy dense actuator assembly suitable for use with a RAT assembly having adequate force capacity approximately mid-way through an actuator stroke, be lightweight, and not require a larger actuator volume than conventional designs.